Distribution and Habitat Preferences of Some Intertidal Amphipods in Iceland

ACTA NATURALIA ISLANDICA ISSUED BY THE ICELANDIC MUSEUM OF NATURAL HISTORY (NATTORUFRA".BISTOFNUN ISLANDS) The :Museum has published two volumes of Acta N aturalia Islandica in the period 1946-1971, altogether 20 issues. From 1972 each paper will appear under its own serial number, starting with no. 21. ACTA NATURALIA ISLANDICA is a series of original articles dealing with botany, geology and zoology of Iceland. ACTA NATURALIA ISLANDICA WIll be published preferably in English, and will appear at irregular intervals. ACTA NATURALIA ISLANDICA may be obtained: 1) on basis of institutional exchange from Museum of Natural History, P. O. Box 5320, Reykjavik, Iceland. 2) by purchase (charges including mailing costs) from Snaebjorn Jonsson, The English Bookshop, Hafnarstraeti 4 & 9, Reykjavik, Iceland. Distribution and habitat preferences of some intertidal amphipods in Iceland AGNAR INGOLFSSON Institute of Biology, University of Iceland, Reykjavik. Abstract. A total of 801 samples of amphipods from 71 sampling stations on all coasts of Iceland were analysed. Results are presented for eleven species belonging to the families Gammaridae and Tali tridae. Three species have a limited distribution around the coasts of Iceland, while the remaining species are found more or less commonly on all coasts where suitable shores exist. The distributional patterns observed were compared to those shown by the same species on the western and eastern coasts of the Atlantic. It is concluded that temperature is an important factor controlling the distribution of these amphipods. Habitat partitioning was analysed by use of 41 habitat categories under eleven main headings. Each of the eleven species was found to have a habitat distribution different from that of the other species, with the possible exception of one species pair. Spatially successional series of species could be distinguished along environmental gradients found from sheltered shores with luxuriant fucoid vegetation to exposed shores without fucoids, from high to low tidal levels on rocky seashores, from upper to lower reaches of estuaries and from high-salinity to low-salinity ponds and lagoons. It is probable that habitat partitioning among the species results from differences in adaptations to several interacting physical factors, of which salinity, humidity, temperature, and oxygen level can be reasonably identified. Interspecific competition may reduce habitat overlap among species but the data are inconclusive. 2 AGNAR INGOLFSSON TABLE OF CONTENTS Introduction 3 Methods 3 Collection of data 3 Analysis of data 4 Results 6 Gammarus oceanicus SegerstriUe 6 Gammarus locusta (Linne) 8 Gammarus setosus Dementieva 8 Gammarus zaddachi Sexton 9 Gammarus duebeni Lilljeborg 9 Marinogammarus marinus (Leach) 15 Marinogammarus obtusatus (Dahl) 15 Marinogammarus stoerensis (Reid) 17 Marinogamm~rusfinmarchicus (Dahl) 17 H yale nilssoni(Rathke) 18 Orchestia gammarellus (Pallas) 18 Association of species 19 Discussion 20 Distribution 20 Habitat preferences 23 Acknowledgements 25 References 25 INTERTIDAL AMPHIPODS IN ICELAND 3 INTRODUCTION spring-tides along transects worked down seashores and along estuaries. Generally 5 - 7 Intertidal amphipods of the families Gam samples were taken along each transect from as maridae and Talitridae in the North Atlantic high up as amphipods were found down to the region have been the subject of many ecological sublittoral. Additional samples were obtained studies in recent years (e.g. Goo d h art 1941, from a variety of habitats whenever these were Spooner 1947, Jones 1948, Steen 1951, encountered. The zonation pattern of each tran Den Hartog 1963, 1964, Vader 1965, sect was described with reference to dominant Rygg 1972, Dennert 1973, Steele and organisms, and on each sampling site the sub Steele 1975c, Van Maren 1975). Habitat strate was described, cover of species of algae partitioning among species has been dealt with in estimated, presence of other animals noted, and many of these studies, although there is a lack of where appropriate, temperature and salinity quantitative data on this aspect, and general measured, the latter by use of a hydrometer with statements are the rule. The present study was an accuracy of 0.2%0. The size of the sampling undertaken in an attempt to place habitat selec sites varied, but was almost always less than 5 m 2• tion of these amphipods on a more quantitative The area searched at each sampling site was basis than had been done previously. There is in reasonably homogenous. A number- of samples addition a unique assemblage of species found on from southwestern Iceland were obtained from Icelandic seashores, the study of which could be sampling sites 800 cm2 in area so that densities expected to yield p~rticularily interesting infor could be determined, but otherwise the aim was mation on niche partitioning among related to obtain at least 100 amphipods in each sample, sympatric species. but.this was frequently not possible. Tidal levels A second aim of this study was to delimit the of many sampling sites in southwestern Iceland range of the intertidal species. As the species in were measured with surveying instruments and question are largely confined to the intertidal with reference to the tidal gauge of Reykjavik zone and therefore have an almost linear dis harbor. The amphipods were picked up from tribution it was thought possible to delimit the under stones or between algae with forceps. The range of each species rather accurately and thus algae were shaken in order to dislodge the facilitate a consideration of environmental fac animals. Sampling from under water was aided tors controlling distribution. Most studies of dis by use of hand nets (mesh-size about 1.5 mm) and tributional patterns of marine animals around on tidal flats the substrate down to a depth of 10 Iceland have been marred by a very irregular cm was sieved (mesh-size 1.5 mm). Almost all sampling effort and there was clearly a need for a samples were obtained from May to August in study based on samples distributed evenly 1974 and 1975. around the coasts of Iceland. Additional unpublished data have been used when appropriate, but separately from those of the general survey. The relevant information is METHODS contained in a number of reports compiled by the author and other staff members and students at Collection of data the Institute of Biology, University of Iceland. Amphipod samples were obtained from all coasts of Iceland. Samples were taken during low 4 AGNAR INGOLFSSON Ana'lysis of data category (10). The rocky seashore is divided into In analysing habitat preferences 41 habitat four types, habitat categories 5 - 8, often referred categories under 11 main headings were esta to hereafter as shore types 5 - 8. The distinguish blished. The categories were defined prior to ing feature of these types is the species composi sampling but somewhat revised after sampling tion of fucoid algae, and they represent a series was completed. Definitions are given in Table 1. from sheltered shores with stable substrate (type 5) Habitats of categories 1-4 are characterized by to exposed shores where considerable movement low and often fluctuating salinities. The lagoons of substrate occurs (type 8). Tidal flats (habitat and coastal ponds (habitat category 1) are of category 10) are of several different types dep several different types, but can be roughly ending on substrate and fresh-water influence, grouped into two groups, 1.1 and 1.2, the former but as few samples were obtained from this receIvmg salt water through underground habitat, the category was not subdivided further. seepage from the sea or via sea-spray, the latter A few samples that could not be placed in a connecting with the sea through a narrow chan habitat category as defined were placed in cate nel. Estuaries (habitat category 3) in Iceland are gory 12 (undefined). usually very short, and pronounced salinity The amphipods of each sample were identified changes are confined to a small area around the and counted under a dissecting microscope. mouth of the river. The term esturary will here be Descriptions and keys by S e g e r s t r a I e used to include the river channel itself traversing (1947), Sexton (1942), Sexton and the intertidal region, as well as the intertidal area Spooner (1940), Stephensen (1928, to each side of the channel if there are no tidal 1935-1942) and S t 0 c k (1967) were the chief flats, while tidal flats are placed in a separate aids used in identification together with a Table 1 Designations and definitions of habitat categories, and number of samples examined from each category from southern + western and from northern + eastern Iceland. Number of samples S + W N + E 1. Lagoons and coastal ponds 31 31 1.1. Little or no tidal change, salinities usually lower than 12%0 1.1.1. Above water level 7 3 1.1.2. Below water level 11 19 1.2. Tidal range noticable, salinities fluctuating but rarely exceeding 25% 1.2.1. Upper shore 3 4 1.2.2. Lower shore 3 5 1.2.3. Below water level at low tide 7 0 2. Tidepools 6 18 2.1. Upper shore, above fucoids when these present 7 2.2. Middle shore, between uppermost fucoids and uppermost Fucus dis tiehus L. when present 4 9 2.3, Lower shore, below uppermost F. distichus when this species present 1 2 3. Estuaries 11 73 3.1. Upper tidal reaches, in river channel 4 22 3.2. Upper tidal reaches, above river channel 2 11 3.3. Lower tidal reaches, in river channel 2 21 3.4. Lower tidal reaches, above river channel 1 19 INTERTIDAL AMPHIPODS IN ICELAND 5 Number of samples S + W N + E 4. Small streams traversing shore 5 13 4.1. Upper shore, above fucoids when these present 3 6 4.2.

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